Wires shell completion for all 4 shells via Cobra's built-in generators.
Covers the remaining item from the implementation status checklist.
Adds:
- internal/cmd/completion.go: cobra.GenXxxCompletion wrappers with
usage examples for each shell
- 5 new integration tests in cmd/molecule/molecule_test.go:
- TestCLI_Completion_Help (4 shells × help flag)
- TestCLI_Completion_GeneratesScript (4 shells × script output)
- TestCLI_Completion_InvalidShell (exit code 2 on bad shell)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
